Polyvinyl acetal resins are used for many purposes such as interlayer films for laminated glass, wash primers for metal treatment, various coating compositions, adhesives, resin processing agents, and ceramic binders. Recently, the applications of polyvinyl acetal resins have been expanded to electronic materials. In particular, laminated glass including a polyvinyl acetal resin film as an interlayer film is favorably used because, even when the glass is damaged by collision of a flying object such as stones, the film between glass sheets absorbs a shock of the collision to prevent penetration of the flying object.
The mechanical strength of thermoplastic resins such as polyvinyl acetal resins, however, commonly has temperature dependence. At low temperatures, though having great breaking strength, a polyvinyl acetal resin has a small elongation rate and is fragile. At high temperatures, in contrast, though having a greater elongation rate, the polyvinyl acetal resin has poor breaking strength and is too soft. In either case, the mechanical strength of the polyvinyl acetal resin is significantly lowered.
Patent Literature 1 discloses, as a polyvinyl acetal resin having better mechanical strength at low temperatures, a laminate wherein a plurality of polyvinyl butyral resin layers is laminated. The laminate disclosed in Patent Literature 1 includes an inner layer that comprises a relatively hard polyvinyl butyral (PVB) and has a Tg within a range of about 35 to 60° C. and outer layers laminated on both faces of the inner layer, which comprise a relatively soft PVB and have a Tg within a range of about 32 to 35° C.
Patent Literature 2 discloses a method for improving mechanical properties of polyvinyl acetal at low temperatures by mixing with a modified polyvinyl acetate having a carboxyl group in the side chain.
Patent Literature 3 discloses a modified polyvinyl acetal having a high glass transition temperature as a polyvinyl acetal resin having better mechanical strength at high temperatures. Patent Literature 3 discloses a modified polyvinyl acetal obtainable by acetalizing a modified polyvinyl alcohol that is prepared by saponification of a vinyl acetate copolymer resulting from copolymerization of 1-methyl vinyl acetate and vinyl acetate.
Patent Literature 4 discloses a modified polyvinyl acetal resin obtainable by acetalizing a modified polyvinyl alcohol randomly having ethylene as a structural unit of a main chain.
Though being capable of improving the mechanical strength at high or low temperatures, the techniques disclosed in Patent Literature 1 to 4 have difficulties in providing a polyvinyl acetal resin with excellent mechanical strength over a wide temperature range from low temperatures to high temperatures.